For many applications it is necessary to measure angles which have an angular separation of more than 360°, that is to say that it is necessary to perform more than one full revolution in order to pass from one angle to the other angle. Examples of such applications are found e.g. in automotive technology if the current steering wheel position is intended to be determined for an electric steering system or servo steering. Further applications are found for example in robot technology and in lifting technology (cranes, elevators, forklift trucks, etc.), in which e.g. a cable drum or winch can rotate over a range of a plurality of revolutions.
In the case of a rotation angle sensor that yields an absolute rotation angle as an output value, the absolute rotation angle is typically determined directly on the basis of a current configuration or internal position of the rotation angle sensor. In contrast thereto, differential rotation angle sensors or relative rotation angle sensors measure the absolute value and, if appropriate, the direction of a rotational movement, such that generally it is necessary firstly to move to a known reference angle, from which other angles can be determined by a difference formation. However, even small inaccuracies add up in the relative rotation angle determination, particularly if multiple revolutions are carried out.
Many absolute angle sensors currently used have a permanent magnet, which is fixed to an end of a shaft, and a magnetic field sensor positioned in an extension of the shaft axis. When the shaft rotates, the magnetic field of the permanent magnet also rotates relative to the magnetic field sensor, which is itself stationary. The sensor detects the magnetic field and derives the rotation angle therefrom. Such systems can use horizontal Hall sensors (HHall devices) or vertical Hall sensors (VHall devices) or magnetoresistive sensors such as GMR sensors (giant magneto-resistors) or AMR sensors (anisotropic magneto-resistors) or more exotic XMR sensors (wherein the abbreviation XMR serves as an umbrella term for the technical know-how based on the magnetoresistance effects AMR, GMR, TMR, CMR and GMI) or combinations thereof. They can detect angles of between 0° and 360°. With absolute magnetic angle sensors of this type it is difficult to create an arrangement having a through shaft, i.e. if the sensor has to be positioned eccentrically with respect to, instead of on or in an extension of, the axis of rotation.
Many differential or relative magnetic angle sensors currently used have a measuring wheel having a multiplicity of permanent-magnetized north and south poles or are made from soft-magnetic material having teeth and notches. The sensor element(s) is/are positioned near the circumference of the wheel (slightly offset with a gap either in a radial direction or in an axial direction; this is called an air gap). All the abovementioned sensor technologies can also be used for these types of rotation angle sensors. They usually detect fluctuations of the magnetic field when the wheel rotates. An algorithm detects the extrema of this pattern and derives switching points therefrom: the switching points are typically average values of preceding maxima and minima. If e.g. 60 north and south poles (i.e. p=60 pole pairs) are arranged along the circumference of the wheel, the sensor angle can resolve increments of 360°/60/2=3°, that is to say a half-period. In principle, it is possible to interpolate between each increment, since the field variation is typically sinusoidal. However, the absolute angular accuracy is often reduced by inaccurate magnetization patterns or insufficient sensitivity of the sensor element or insufficient robustness toward external magnetic disturbances in the case of large air gaps (in particular for small magnetic domains, which means a large value for the number of pole pairs p).
In all the abovementioned cases of absolute angle sensors, the range of the detectable angles is at most 360°. If a plurality of revolutions are intended to be detected, some rotation angle sensors currently used use gear mechanisms which copy the rotation of the shaft onto two shafts having different rotational speeds. Each of the two shafts has its own magnet with its allocated sensor and both sensor signals are compared in order to determine the absolute angle. If the transmission or reduction ratio of the two magnets is 1:5, for example, it is possible to detect angles in a range of 5*360°, but if the transmission or reduction ratio is large, even small errors of the two angle sensors can produce large errors in the absolute angle, which can reach a multiple of 360°. Such systems are bulky and costly on account of the gear mechanisms and it is typically not possible to make them very small, since this would lead to a magnetic crosstalk between the two sensor systems, for which there is no simple possibility of correction.
It would be desirable to have available an absolute angle sensor which can be mounted in configurations with a through shaft and which can detect angular ranges of up to a plurality of revolutions. It would also be desirable to provide a rotation angle sensor which makes it possible to ascertain or monitor whether a tolerance range relevant to an accuracy of the rotation angle measurement has been complied with or departed from.